Moving cavity motors or pumps, sometimes known as positive displacement motors or pumps, or progressive or progressing cavity motors or pumps, work by trapping fluid in cavities. The cavities are formed in spaces between the rotor and the stator, and the relative rotation between these components is the mechanism which causes the cavities to progress and travel axially along the length of the device from the input end to the output end. If the rotor is forced to rotate, fluid is drawn along in the cavities and the device will be a pump. If the fluid is pumped into the input end cavity at a higher pressure than that at the outlet end, the forces generated on the rotor cause it to rotate and the device will be a motor.
In order that the rotor can rotate within the stator and generate cavities that will progress in an axial direction, the profiles of both components must take specific forms. Typically, the rotor (2) will be a helically shaped shaft with a sectional shape similar to those shown in FIG. 1. The number of lobes on the rotor (2) can vary from one to any number. The stator (4) has a profile which complements the shape of the rotor (2), with the number of lobes varying between two and any number, examples of which are illustrated in FIG. 2. In a matching rotor-stator pair, the number of lobes on the stator (4) will be one greater than on the rotor (2). A section through a typical combination of rotor (2) and stator (4) is shown in FIG. 3, in which the rotor (2) has three lobes and the stator (4) has four lobes, with the rotor (2) being received within the stator (4).
One of the surfaces, often that of the stator (4), is flexible so that seals (6) can be maintained between the points of contact of the rotor (2) and the stator (4). The seals (6) define a plurality of cavities (8) between the rotor (2) and the stator (4) and still allow for relative rotation between the rotor (2) and stator (4). The rotor (2) and stator (4) sections typically remain the same along the length of the motor or pump (10), but progressively rotate to result in a helical profile. A section through a diametral plane of part of a motor or pump (10) is shown in FIG. 4.
The rotor (2) does not have to be of a fixed length. The chosen length is often defined in stages where one stage consists of a complete rotation of the helix of the stator (4). The cavities (8) are formed between the stator (4) and the rotor (2).
It will be apparent from the sections in FIG. 3 and FIG. 4 that the geometric centre of the rotor (2) does not remain fixed relative to the stator (4) as the rotor (2) turns. Generally, where the rotor (2) has two or more lobes, the trajectory of the centre point is roughly a circle, with variations caused by the exact nature of the surface profiles and any deformations in the flexible materials used to maintain the inter-cavity seals (6). Both in the case of a motor, where the rotor (2) provides the driving torque, and for a pump where the rotor (2) is driven, a drive shaft assembly (12) is required to transform a rotation about an orbiting axis to a rotation about a fixed axis. This drive shaft assembly (12) has a moveable joint assembly (14) to facilitate this mechanism. In the case of a motor, the outside end of the drive shaft (13) is connected to the component that requires to be driven, a drill bit for example in the case of a downhole motor. For a pump, the outside end of the drive shaft (13) is connected to a source of rotational energy such as a motor.
The torque that is generated in the rotor (2) in the case of the device being a motor, or required in the rotor (2) in the case of the device being a pump, is a complex combination of the pressure forces acting in the cavities (8) and the reaction forces between the points of contact between the stator (4) and the rotor (2). This has the effect of trying to turn the rotor (2) in the case of a motor or resisting rotation in the case of a pump. In both cases there is also a net lateral force that acts to push the rotor (2) into the stator (4). The direction of this force rotates as the rotor (2) turns. There is also a centrifugal force generated by the orbital motion of the rotor. And in the case of a motor, such as a mud motor, there may be a lateral component of the thrust carried by the transmission.